There are no changes from Matthew 6:16–18.
Jesus returns to examples of how humankind might perform religious labors to be noticed by others rather than because of their devotion to God. In this case, the subject is fasting. It is assumed that fasting is important and should be done. The difference, as with the other examples, is for whom one is fasting. That is, do we fast to improve our relationship to God, or to humankind. If we perform the tasks specifically so others will see and recognize how righteous we are, then we are not truly righteous.
The theme is repeated that those who do such things for the accolades of humankind will get the reward they look for, but only that reward. God will reward the heart, not the performance.